I've been doing a lot of negative mode direct infusion on an orbitrap lately. Almost always, the S/N level is awful even if I'm infusing high concentrations, e.g. 100 ÂµM. At first I thought it was because the samples contained some salt but recently I infused a sample which I'm sure did not contain any salt and still got very poor S/N. I haven't had the same trouble with positive mode. I'm wondering if it's because the analyte is not ionizing well due to the lack of anything to extract protons? Do people ever add base to improve the signal in negative mode? If so, what do people add and at what concentration?

Back when I was doing more negative mode direct infusion, I tested a ton of different spray solutions and found that a mix of 50:50 (vol/vol) 5 mM NaOH:2-propanol seemed to work best. Here's a link that describes a bit more of the methods in case it's helpful http://www.ncbi.nlm.nih.gov/pubmed/22290482